1. Field of the Invention
This invention relates in general to controlling vibration and noise in heating and cooling systems and, in particular, to a method for reducing acoustical resonances in compressor discharge lines. More specifically, but without restriction to the particular embodiment hereinafter described in accordance with the best mode of practice, this invention relates to determining the optimum placement of a refrigerant line muffler to reduce suction and acoustical effects caused by formation of a standing wave in compressor suction and discharge lines.
2. Discussion of the Related Art
Prior research has identified three types of refrigerant line vibration. These include structural resonances, forced vibration, and acoustical resonances. Structural resonances result in refrigerant tubing systems when the frequency of the discharge gas pulsations exiting the system compressor is substantially equal to the natural frequency of the system tubing. Forced vibration occurring in refrigerant lines is attributed to compressor movement caused by compressor starts, stops, and continuing operation. Lastly, acoustical resonances are related to refrigerant tubing geometry and specific physical properties of the discharge gas.
It has previously been observed that standing waves caused by these acoustical resonances may form in the discharge line of a compressor employed in a heating or cooling system. Such standing waves contribute to the noise and vibration associated with operation of the system. In some instances, this vibration may be transferred to building ductwork, thereby driving the ductwork and other building elements which results in added system noise.
Established theory has been developed to predict a system critical length, which is a function of the frequency of the discharge gas pulsations, the speed of sound in the discharge gas, and the wavelength of the discharge gas pulsations. This theory holds that standing wave resonances will occur when the calculated critical length of a particular system matches the length of an element of tubing in the system. Under this theory, an element is defined as a segment of tubing between two terminations. Such terminations would include compressors, mufflers, sharp elbows, headers, or any other type of line equipment that causes a change in acoustic response.
Heat pumps are one type of heating and cooling system that have been identified as being subject to the undesirable noise effects of acoustical resonances. The typical heat pump system is comprised of five primary components of equipment including a compressor, a flow restriction valve (cooling and heating), reversing valve, and two heat exchangers. The heat exchangers are customarily referred to as the indoor and outdoor coils. The indoor coil is ordinarily positioned in the building structure within ductwork employed for directing the circulating room air. A fan is typically positioned adjacent to the indoor coil to induce the circulating air to flow over the coil and through the ductwork. The combination of the indoor coil and fan, and the enclosure which houses these components is commonly called the indoor unit. The compressor, the flow restriction valve, the reversing valve, and the second heat exchanger, which is commonly known as the outdoor coil, are typically contained within a single housing unit commonly called the outdoor unit, which is positioned on the exterior of the building structure to be heated or cooled. A fan is provided in the outdoor unit to induce heat exchange between the outdoor coil and the ambient air. These various heat pump elements are connected in a closed series loop by a system of tubing carrying a refrigerant. The direction of flow of refrigerant in the closed system is controlled by the reversing valve. This reversal of refrigerant flow direction allows the heat pump to be quickly switched between its cooling and heating cycles. It has recently been observed that acoustical resonances in heat pump systems occur principally during the heating cycle of the heat pump. The standing wave associated with these acoustical resonances begins formation in the compressor discharge line of the tubing system or network and is reflected through the tubing system between terminations. These acoustical resonances or disturbances can result in annoying low-pitched noise which is amplified by the indoor components of the system. Such components would include, for example, the indoor coil, the tubing kit, which connects the indoor unit to the outdoor unit, and any abutting house structure.
Prior methods for reducing the undesirable effects of acoustical resonances have involved attempting to design a refrigerant line system by avoiding elements of tubing having a length substantially equal to the calculated critical length for the particular system. These methods have resulted in limited success because standing wave patterns attempt formation regardless of element length. This partial formation of the standing wave also produces some degree of acoustical resonances and unwanted low-pitch noise. Other prior methods for reducing the effects of acoustical resonances in refrigerant lines have focused on installing a muffler as close as possible to the service valve or compressor while keeping them out of the critical ranges calculated for standing wave patterns. These methods have also met with moderate success for reasons not well understood prior hereto.